1. Field of Invention
This invention is in the field of Synthetic Aperture Radar (SAR) where the platform acquiring the image deviates from an ideal flight path and the terrain being illuminated has variations in elevation above sea level.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses. These pulses are from a relatively small antenna on a moving platform. As the platform moves, the information contained in the pulses is coherently combined to arrive at a high resolution SAR image.
The plurality of returns creating a SAR image generated by the transmitted pulses are assumed to be acquired along a presumed known, planar and rectilinear path of the platform to make up an array. If following the proper path, during the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. That is, the SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.
The clarity of details of a SAR image is dependent, among other factors, on the accuracy of the motion compensation applied to each radar return prior to SAR image computation as well as relative position of the platform with respect to the ideal path required for SAR image acquisition. Motion compensation shifts the phase of each radar sample (typically an I+jQ complex quantity derived from an analog to digital converter) in accordance with the motion in space of the moving platform with respect to the range to a reference point. The SAR imaging process depends on the coherent, phase accurate summing of the sequence of ALL radar returns expected within an array. These principles are detailed by W. G. Carrara, R. S. Goodman and R. M. Majewski in Spotlight Synthetic Radar, Boston, Artech House, 1995.
However, coherent summing depends on an accurate description of SAR platform (aircraft) path. Where a non-ideal platform motion exists, having flight path deviations (FPD) away from a rectilinear, planar path, motion compensation of the pulses may not totally account for phase errors. Consequently, the coherent summing of radar returns over an array will also be inaccurate. The resulting phase error over the array blurs the SAR image, reducing its utility.
Another limitation of the motion compensation mechanism is the inherent assumption that the area being illuminated is at a constant ground elevation, relatively flat. Where that is not the case, returns will arrive at a range different from the one assumed, thus introducing phase inaccuracies. Thus, variations in elevation bring a resulting SAR image that is again blurred, reducing its utility.